The numbers you need to look for are stamped, not cast, on a machined pad directly behind the distributor. The first three digits will identify the size of the engine. The same bellhousing that bolts to a Chevrolet 216 or 235 engine will fit all the older GMC sixes, 228 through 302. If you want a modern design automatic transmission such as a Turbo 350 or a 700R4, an adapter plate and some modification of the crossmember at the rear of the engine will be required, plus a rear support for the transmission will be needed. If the current rear end uses an enclosed driveline (torque tube) then the rear end would also need to be swapped to accommodate the automatic trans.
Jerry
